In cold chamber die-casting machines the use of injection pistons with a steel or copper body and at least one outer sealing ring fitting in a seat next to the piston head are known of.
An example of such pistons is described in U.S. Pat. No. 5,233,912.
In WO2009125437, in the name of the same applicant, a piston for cold chamber die-casting machines is described comprising a body terminating at the front with a frontal surface pressing the molten metal and at least one sealing ring mounted in a respective annular seat made around said body. At least part of the bottom surface of the seat is crossed by at least two channels which extend mainly in a longitudinal direction and which come out at the front in said frontal surface of the piston for an inflow of the molten metal under the ring.
Preferably, said channels extend from the frontal surface of the piston almost up to the median line of the seat of the ring, so as to bring the molten metal mainly towards the barycentre of the sealing ring 16.
In such a way, the metal flowing to the seat, solidifying, creates a continuous thickening which radially pushes the ring outwards, thus progressively recovering wear, adapting it to any deformation of the piston container and thus protecting the latter.
It has however been experimented that with the piston described above, the molten metal which penetrates the channels does reach a central zone of the ring seat, that is to say deposits mainly under the barycentre of the ring, but, in certain conditions of use, is not always successfully distributed in an even manner around the entire bottom surface of the ring. In other words, in some cases, the metal which comes out of a channel penetrating under the ring does not have sufficient thrust to continue to flow towards the adjacent channels, but tends to solidify only at the end of the channel which it came out of. Consequently, the radial thrust caused by the metal which has flowed under the ring is located mainly in some zones causing an uneven distortion of the ring. The recovering of wear is, as a result, uneven around the ring, and the perfect adaptation of the ring itself to the inner surface of the container, which the piston slides in, is not achieved.
In addition, such distortion of the ring in turn causes a counter-thrust or reaction on the solidified metal below it, which obstructs the flow of new molten metal below that already solidified.
To such purpose, it is to be noted that while in hot chamber die-casting machines the piston is always immersed in a bath of metal in a liquid state, in cold chamber applications, every time the piston is returned to a rearward position and the die opened, the cooling system leads to the formation of a metal riser in front of the frontal surface of the piston and, in the case of the piston described above, to the solidification of the metal which has found its way into the channels and under the ring. One of the difficulties of making a piston recovering wear for cold chamber die-casting such as that described above consists of the fact that if one wishes new metal to flow under the ring at each work cycle to progressively recover wear, then when opening the die to remove the casting the metal which has solidified in the channels must also remain attached to the metallic riser attached to the piece. It is clear that the objective of trapping the metal under the sealing ring, therefore in a rearward position of the frontal surface of the piston as evenly as possible along the circumference of the piston, contrasts with the need to remove the riser so as to liberate the inflow channels of the metal under the ring at each cycle.
For example, it has been seen in some cases, with the piston described above, that the metal which has solidified in the channels is not completely removed together with the metallic riser but remains inside such channels preventing a correct inflow of metal under the ring in the subsequent cycle.
As said, all these problems are not present in hot chamber die casting machines in that the metal which has found its way into any interstices or passages intentionally created or present in the piston, does not solidify.